(1) An electronic component that acts like a one-way valve. As a discrete component or built into a chip, it is used in a variety of functions. Used as a rectifier, it is a key element in changing AC to DC by limiting current flow to a single direction. Diodes are used as temperature and light sensors and light emitters (LEDs). In communications, they filter out analog and digital signals from carriers and modulate signals onto carriers. In digital logic, they're used as one-way valves and as switches similar to transistors. See laser diode and PN junction.

(2) A type of vacuum tube used in electronic circuits as a rectifier or radio frequency detector. Modern applications of tube diodes are generally limited to rectifiers in high-end audio amplifiers and other specialized high-voltage circuits.

The tube diode uses two active elements (cathode and plate) and one passive element (the filament or heater). In typical operation, the cathode is heated by the filament, and the AC voltage is applied to the cathode. The heated cathode releases excited electrons that flow to the plate (anode) and become the rectified current. The diode allows current flow in only one direction. For example, if current were applied to the plate, electron flow could not occur, because the plate's electrons are not heated by the filament.

In some instances, the filament is also the cathode. This is accomplished by connecting the AC voltage source to one of the filament's leads. See triode, tetrode, pentode, magnetron and klystron.